Two types of disc drive are currently well known: a magnetic disc drive wherein information recorded on a magnetic disc is read out by a magnetic head; and, a so-called optical disc drive (including an optical magnetic disc drive) wherein information recorded on a disc such as a compact disc is read out by an optical head.
FIG. 9 illustrates a structure in a periphery of a head carriage of a conventional magnetic disc drive. The magnetic disc drive is a device which is used to drive a magnetic disc such as a flexible disc (not shown). The magnetic disc is inserted into the magnetic disc drive from a direction as shown by an arrow A in FIG. 9. The inserted magnetic disc is held on a disc holder 11 wherein a center axis of each part is matched each other. The disc holder 11 is rotatably supported through a spring 12 on the surface of a frame 13. The disc holder 11 is rotatably driven by a motor (not shown) provided below the frame 13, allowing the magnetic disc to rotate. A printed circuit board (not shown) loaded with many electronic components is also provided underneath the frame 13.
The magnetic disc drive includes a magnetic head 14 for reading/writing data in the magnetic disc. The magnetic head 14 is supported by a head carriage 15. As will be described in the following, the head carriage 15 is positioned apart from the frame 13 on the surface of the frame 13. The magnetic head 14 is movably supported in a predetermined radial direction (as shown in an arrow B in FIG. 9) of the magnetic disc.
A stepping motor 16 is fixed on a side wall 131 of the frame 13. The stepping motor 16 drives the head carriage 15 along the radial direction B. In more details, the stepping motor 16 includes a rotation (drive) shaft 161 extending therefrom in a parallel direction with the radial direction B, wherein the rotation shaft 161 is shaped in a male screw. An end of the rotational shaft 161 is supported by a bearing 132a on a bend 132 cut out from the frame 13. By means of the bearing 132a, the rotation shaft 161 is directed to extend in the parallel direction with the radial direction B and rotatably supported. The head carriage 15 includes a carriage arm 151 extending therefrom to the rotation shaft 161, wherein a pin 151a is provided on an end of the carriage arm 151 so as to engage with a groove of the male screw of the rotation shaft 161.
Therefore, if the rotation shaft 161 of the stepping motor 16 rotates, the pin 151a of the arm 151 moves along the groove of the male screw of the rotation shaft 161. As a result, the head carriage 15 also moves along the predetermined radial direction B. Thus, the stepping motor 16 functions as a driving means for moving the head carriage 15 along the predetermined radial direction B.
One side of the head carriage 15 is movably supported by the rotation shaft 161 of the stepping motor 16 and arranged to be apart from the frame 13. However, it is not sufficient to support an entire head carriage 15 only by the rotation shaft 161. For the other side of the head carriage 15, another supporting and guiding means is needed. For such a purpose, in this example, a guide bar 17 is provided as a guiding means. Namely, the guide bar 17 is provided in the opposite side of the rotation shaft 161 of the stepping motor 16, having the head carriage 15 in the middle. The guide bar 17 is extended in the predetermined radial direction B. Both ends 171 and 172 of the guide bar 17 are fixed on the surface of the frame 13 so as to guide the head carriage 15 in the radial direction B, as will be described in more detail later. Therefore, the head carriage 15 is entirely kept apart from the surface of the frame 13. Further, a lead wire 15b is extended from the head carriage 15 toward the guide bar 17. The lead wire 15b is connected to the printed circuit board mounted underneath the frame 13.
The guide bar 17 is clamped by a clamp 18 on the surface of the frame 13. The clamp 18 is fixed on the surface of the frame 13 by a screw 19. The clamp 18 includes clamping parts 181 and 182 on its both ends for clamping the both ends 171 and 172 of the guide bar 17.
In this type of conventional magnetic disc driver, the following inspection and adjustment are performed with respect to the positional shift of the magnetic head. For example, a 3.5 inch magnetic disc which commonly contains 80 tracks is adjusted so that the magnetic head transmits a so-called cat's eye signal as shown in FIG. 10A when the magnetic head is positioned in a track, for instance, the 40th track. Namely, when the symmetrical pattern of the cat's eye signal as shown in FIG. 10B is obtained, it means that a reading/writing gap of the magnetic head is positioning in the center of the predetermined track of the disc.
The above operation is preferably performed under such environment where the ambient temperature is around 20.degree. C. However, in reality, the disc drive may be used under other conditions where the ambient temperature is higher than 50.degree. C. or lower than 0.degree. C. It is a characteristic of the magnetic disc, the frame 13 and the head carriage 15 that these components are expanded under such higher temperature and shrunken under the lower temperature. This temperature change causes a positional shift of the magnetic head, which is called a thermal off-track. For example, the position of the magnetic head shifts to the inner side of the track if the temperature is higher than 50.degree. C., causing the asymmetrical cat's eye signal as shown in FIG. 10B. If the temperature is lower than 0.degree. C., the position of the magnetic head shifts to the outer side of the track, causing the asymmetrical cat's eye signal as shown in FIG. 10C.
Another type of a magnetic disk drive which is similar to that of FIG. 9 is shown in FIG. 11. This magnetic disk drive system 250 possesses a carriage 220 whose main component is made of composite resin and has a side-opening shape which can cover a magnetic disk D that is to be rotated by a rotational table 212. At this carriage 220, a movable magnetic head 224 is provided to an upper portion 221 which is designed to move up and down. Also, a fixed magnetic head 225 is provided to a lower portion 222 of the carriage 220. The magnetic disk D is inserted between the movable magnetic head 224 and the fixed magnetic head 225, and is guided by a guide rod 214 so that the magnetic disk D can move in its rotational direction.
As shown in FIG. 11, at the side of carriage 220, a rotation shaft 218 having a screw and rotatably driven by a motor 217 is provided in a manner to be parallel with the moving direction, i.e., in parallel with the above mentioned guide rod 214. Also, at the side of the carriage 220, there is integrally provided an arm 240 for orthogonally coupling the rotational shaft 218 to a side of the carriage 220. The arm 240 has a pin 241 at its tip such that the pin 224 engages the screws of the rotation shaft 218.
In this conventional magnetic disk drive, a relatively large space L is required between the rotational shaft 218 and the carriage 220 because of the reasons determined by the layout of various components. Thus, the arm 240 is required to be relatively long. In this structure, there is a possibility that the carriage 220 sways along the moving direction as shown in an arrow S in FIG. 12 because of the influence of conditional changes such as temperature and humidity, since the arm 240 is made of composite resin integral with the carriage 220 and has a cantilever form. If the arm 240 sways like this manner, memory retrieval operations with respect to the magnetic disk will be adversely affected (such as an increase in the absolute value of a thermal off-track). In such a situation, it may be possible to utilize metal of high rigidity for the arm 240, however, such a use of metal in the arm will increase the cost of the disk drive.